The present invention relates to law enforcement technologies and security, and more particularly to methods and systems for determining the origin and direction of travel of supersonic projectiles based on shockwave and muzzle sound information. The methods and systems are capable of determining and disambiguating shooter locations even for large distances between shooter and sensor and when the signal from the muzzle sound is weak.
Systems and methods are known that can determine the general direction and trajectory of supersonic projectiles, such as bullets and artillery shells by measuring parameters associated with the shockwave generated by a projectile. One such system, described in U.S. Pat. No. 5,241,518 includes at least three spaced-apart sensors, with each sensor incorporating three acoustic transducers arranged in a plane. The sensors generate signals in response to the shockwave which are related to the azimuth and elevation angle to the origin of the shockwave. However, shock-wave-only measurements are unable to determine the distance between the sensor(s) and the origin of the shockwave. With additional information from the muzzle blast, an accurate location of the origin of the projectile and a line of bearing to the origin of the projectile can be determined. However, when the muzzle blast is masked, shadowed or otherwise distorted, the signal from the muzzle blast may be difficult to discern from spurious signals and noise, so that the derived distance information may become inaccurate. Moreover, even if a clearly discernable muzzle blast signal were received from distant shooter locations, determining the range (distance) of the shooter can still remain a problem.
Conventional systems employ acoustic sensors, e.g., microphones, which can be relatively closely spaced (e.g., 1 meter apart) or widely dispersed (e.g., mounted on a vehicle or carried by soldiers on a battlefield), and measure shockwave pressure omni-directionally at their respective locations.
The azimuth angle, elevation angle, and range of a shooter with reference to the sensor location can be determined by measuring Time-of-Arrival (TOA) information of the muzzle signal and shockwave signal at each sensor. Each of the sensors encounters these signals at a different time and generates an electric signal in response to the shockwave pressure. The signals from the various sensors are processed, and a direction (azimuth and elevation) from the sensor(s) to the origin of the shockwave as well as the range, and hence the trajectory of the projectile can be determined.
Conventional algorithms require at least 4 shockwave and muzzle detections so that a 4×4 matrix can be inverted to map a plane wave on the shockwave TOA. Small errors in shock and muzzle TOA determination can produce substantial errors in the range estimations. Moreover, the conventional algorithms assume a constant bullet speed along the bullet trajectory, which gives inaccurate range estimates for long-range shots being fired from a distance of more than approximately 300 m.
Accordingly, there is a need for rapidly converging algorithms capable of accurately estimating a distant shooter range.
In addition, there is a need to disambiguate shock-wave only solutions for the shooter direction. This is typically accomplished by using the muzzle signal. Disadvantageously, however, less than 4 muzzle sounds may be detected in many situations, for example, due to extraneous noise, reflections, etc., that can mask the muzzle signal. It would therefore be desirable to provide a method for reliably disambiguating shock-wave only solutions even if muzzle blast signals are detected on less than 4, for example 2 or 3, acoustic sensors.
In some situations, an initially detected muzzle signal may be discarded as being unreliable, for example, because the detection level is too low to indicate a muzzle blast signal; or because the muzzle energy is not readily evident in the raw signal; or because echoes from the shockwave are stronger than the muzzle blast and arrive earlier than the actual muzzle blast, causing the detection system to falsely identify shock as muzzle. These uncertain muzzle blast signals, if properly identified and extracted, may still be useful for refining detection of the shooter position, in particular in conjunction with the detection of shock-wave signals.
It would therefore also be desirable to provide a method for extracting muzzle signals that may be obscured by acoustic signatures unrelated to the muzzle blast.